The present invention relates to the field of the determination of damage to a geological formation element suitable for modelling a rock type material subject to structural changes such as deformations, cracking, fragments with interactions between the fragments, within the framework of the investigation and operation of hydrocarbons, in liquid or gas form from kerogen pyrolysis.
The material modelled is for example a bituminous schist which is a heterogeneous material consisting of mother rock (also known as kerogen) and minerals. The kerogen may be led to undergo pyrolysis, natural or induced, during which it is particularly converted into hydrocarbons, in liquid of gas form, associated with other compounds such as CO2, H2O, etc. Such phase changes of the kerogen, essentially by releasing pressurised gas, generate considerable stress in the material and are a source of deformations, cracking and fragmentations of the material.
Modelling the damage of a material subject to such structural changes in a given geological formation is useful in the context of the investigation and production of hydrocarbons or gas in bituminous schists as such modelling makes it possible to estimate the effect of induced pyrolysis on the kerogen contained in the formation, and as such the available resources associated with said formation, the future production and, therefore, the economic value of the field.
There are a number of approaches for modelling the behaviour of materials under stress that can be grouped into two major categories: continuous approaches and discrete approaches.
Continuous approaches account for the behaviour of the different phases of the material and the physics of the problem in the form of behaviour laws formulated in partial derivative equations. Finite element resolution methods are used to solve these equations.
The drawbacks of continuous approaches are numerous when it is sought to apply them to heterogeneous materials such as bituminous schists. These approaches require an extremely fine mesh to be able to account for the microstructural heterogeneities of the bituminous schists and thus a considerable computing time. Furthermore, behaviour laws in finite elements are not suitable for correctly addressing the problem of damage to materials once phenomena such as crack opening and propagation have a predominant role, which is particularly the case during the structural change of bituminous schists.
Discrete approaches have been developed with a view to modelling non-cohesive divided materials such as granular materials. In these methods, the material is described as a collection of rigid and independent bodies, particles, which interact with one another by means of force laws linking the contact force with a deformation variable associated with contact (defined by means of rigid degrees of freedom of the particles). The change to the system is then obtained by integrating motion equations. The interactions in question, such as cohesion, contact or friction, differ according to the method, the layouts thereof and the target application. Discrete approaches are grouped under the term “DEM”, standing for “Discrete Element Method”, by opposition with finite element methods.
“DEM” methods are generally unsuitable for modelling continuous materials. Indeed, particle deformation and damage are not taken into consideration and crack propagation merely takes place by the rupture of cohesive links and by bypassing particles. Furthermore, the overall behaviour of the material is the result of a large number of interactions on a particulate scale. The mechanical properties of the material are thus resultant properties and cannot be introduced directly into the discrete formulation as in the case of continuous methods. Moreover, in three dimensions, the use of spherical particles offers optimum numerical performance due to the ease of management of contacts but introduces an artificial void phase into the material. The use of other forms of particles requires the management of more complex contacts and thus a greater computing time. Finally, the initial geometric configuration of the discrete domain can have a great impact on the mechanical behaviour of the material and phenomena such as crack propagation along preferential paths or non-uniform diffusion of elastic waves can be observed.
A third approach has been proposed to overcome these drawbacks, referred to as the lattice model. In this approach, the material is not represented by a set of volume elements, as in the continuous approach, or by a set of extended rigid bodies in contact, as in the discrete approach, but by a distribution of points, nodes, interlinked by interaction laws, links. Only local laws, such as the equilibrium of forces and moments, are taken into consideration, and the implementation thereof is performed at each node linked to a limited and defined number of adjacent nodes.
This numerical approach makes it possible to model the phases of the material and optionally the interfaces thereof on a lattice of one-dimensional elements, links, which may have different rheological behaviours. The complexity of the microstructure and the behaviour of the phases thereof are thus taken into account.
The links linking the nodes make it possible to model the mechanical behaviour of the different geological phases of the material and usually have a fragile rheological behaviour characterised by a rigidity linked with the elastic modulus of the geological phase in question and a force rupture threshold linked with the rupture stress of said geological phase.
Crack formation in the geological model is then taken into account through the elimination of links between nodes when a force applied to a link is greater than a force threshold value of the link.
However, this method has drawbacks and is not suitable for modelling in a satisfactory manner damage to the geological model beyond the mere opening of a crack. As such in particular, once a link has been broken, the two nodes originally linked by said link, and forming the two lips of the crack, do not interact with one another. This method is thus not suitable for simulating crack propagation, material deformations, multiple crack formations and fragmentations with interactions between the material fragments.
There is thus a need for a method for determining damage to a geological model capable of accounting for microstructural heterogeneities, reliably modelling the deformation of the geological phases of the material, and simulating phenomena such as the opening and propagation of cracks which play a predominant role during the structural change of bituminous schists.
The present invention improves the situation.